1. Technical Field of the Invention
The present invention relates to pressure sensing apparatus, for example for sensing pressure in a fluid extraction well such as a hydrocarbon extraction well.
2. Background of the Invention
FIG. 1 shows, diagrammatically in section, typical existing pressure sensing apparatus as an assembly 1 fitted in a hydrocarbon production fluid extraction well. The assembly 1 is typically attached to the wall of a fluid extraction pipe 2 via a flange 3. In order to prevent the fluid flow of wellbore product whose pressure is to be sensed from contaminating and blocking the pressure sensing aperture, a flush diaphragm 4 is fitted over the aperture, the diaphragm hydraulically connecting to a remote pressure sensor 5, in the example a quartz device, via a hydraulic, oil filled, very small bore, tubing 6 and hydraulic oil in a cavity 7 defined by a funnel 8. The pressure sensor 5 is located close to an electronic unit 9 which contains an oscillator for the quartz device and converts the electrical output of the pressure sensor 5 into a form suitable for transmission remotely from the assembly 1. The hydraulic tube 6 connecting the diaphragm 4 to the pressure sensor 5 is typically fitted with a facility 10 to evacuate it and permit filling the tube with hydraulic fluid. The pressure sensor 5 and electronic unit 9 are mounted at the cool end of the assembly 1 (that is in a cool environment, e.g. sea water, as compared with the inside of pipe 2), in order to maximise the life of the assembly.
A problem with such pressure sensor assemblies is that there is a differential thermal expansion between the tube 6 and the hydraulic fluid in the tube, causing errors in pressure measurements. Although some attempt is often made to compensate for this by adjustment in the electronics, this is generally inadequate, particularly under transient conditions, leaving significant errors in pressure measurement.